Thickeners are useful for decorative and protective coatings, paper coatings, cosmetics and personal care items, detergents, pharmaceutical, adhesives and sealants, agricultural formulations, petroleum drilling fluids, and the like.
Thickeners have several roles in aqueous systems. They increase viscosity and maintain viscosity at required levels under specified processing conditions and end use situations. In latex decorative coatings, for example, the thickener may provide improved stability, pigment suspension, and application properties. In cosmetics and personal care items, the thickener improves body, smoothness and silkiness, making the product more aesthetically pleasing. In petroleum drilling fluids, the thickener improves the suspension of the cuttings, increasing the efficiency with which they can be removed.
Many thickeners, both natural and synthetic, are known. Natural thickeners, for example, include casein, alginates, gum tragacanth, and modified cellulose, including methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, and carbomethoxy cellulose. These natural products vary in their thickening efficiency, and generally provide poor flow and leveling properties. They are also subject to microbial attack which requires the additional presence of antimicrobial agents. Synthetic thickeners include various acrylic polymers and maleic anhydride copolymers. Some of these are found to be pH dependent, others are hydrolytically unstable, and others are sensitive to various components normally found in aqueous coatings.
One type of synthetic thickener is a polyurethane. U.S. Pat. No. 4,079,028 discloses polyurethane thickeners having at least three hydrophobic groups, such as hydrophobic isocyanate groups, interconnected by hydrophilic polyether groups. These polyurethanes have terminal hydrophobic groups.
Aqueous coating compositions thickened with polyurethane thickeners have good flow and leveling. "Leveling" as used herein, refers to the degree to which a coating flows out after application so as to obliterate any surface irregularities such as for example, brush marks, "orange peel", peaks, or craters, which have been produced by the mechanical process of applying a coating. Thus, aqueous coatings thickened with polyurethane thickeners have a desirable, smooth appearance when dried.
Despite these advantages, aqueous coatings thickened with polyurethane thickeners require improvement in their resistance to sagging. "Sagging", is the downward movement of a coating on a vertical surface between the time of application and setting, resulting in an uneven coating having a thick bottom edge. The resulting sag is usually restricted to a local area of a vertical surface and may have the characteristic appearance of a draped curtain. Sagging is aesthetically undesirable. In addition, coatings which resist the tendency to sag will not easily drip off a paint brush or a paint roller and will not easily drip off a horizontal surface, such as for example, a ceiling.
There is a need for a polyurethane thickener that possesses good thickening efficiency and desirable sag resistance.
According to a first aspect of the present invention, there is provided a mixture of polyurethanes comprising a first polyurethane with at least two end groups, where each end group comprises a terminal isocyanate and a polyether; a second polyurethane with at least two end groups, where each end group comprises a terminal isocyanate group and a non-functional group; and a third polyurethane with at least two end groups, where one end group comprises a terminal isocyanate and a polyether and one other end group comprises a terminal isocyanate and a non-functional group.
A second aspect of the invention is an aqueous composition comprising from 0.005 to 20 percent by weight of this polyurethane mixture.
A third aspect of the present invention is directed to a method of improving the sag resistance of an aqueous composition by adding this polyurethane mixture at a concentration of from 0.005 to 20 percent by weight of the aqueous composition.
The polyurethane mixture of this invention is particularly advantageous for use in latex coating compositions, especially in paints. While it is useful for increasing the viscosity of an aqueous composition, the most important advantage is the sag resistance it imparts. Aqueous compositions thickened with the polyurethane mixture of this invention are structured and solid-like, characteristic of a gel. The gel structure generated by the polyurethane thickener is desirable because aqueous compositions with gel structure resist the tendency to sag. In addition, aqueous compositions with gel structure do not drip easily off a paint brush or paint roller. A further advantage of the improved polyurethane mixture of the present invention is that it is resistant to microbial attack and incorporates easily in aqueous compositions. In addition, the polyurethane of this invention is also advantageous because it can be used as a cothickener with other thickeners to obtain an aqueous composition which does not sag and has a desirable balance of other properties, such as for example, flow and leveling.
This invention is directed to a mixture of polyurethanes. Each of the polyurethanes in the mixture may be present in an amount ranging from about 5 to about 90 mole percent. More preferably, the first polyurethane is present in the mixture in an amount ranging from about 8.3 to about 75 mole percent, the second polyurethane is present in the mixture in an amount ranging from about 8.3 to about 75 mole percent, and the third polyurethane is present in the mixture in an amount ranging from about 16.7 to about 83.4 mole percent. Even more preferably, the first polyurethane is present in the mixture in an amount ranging from about 8.3 to about 25 mole percent, the second polyurethane is present in the mixture in an amount ranging from about 25 to about 75 mole percent, and the third polyurethane is present in the mixture in an amount ranging from about 16.7 to about 50 mole percent. Most preferably, the first polyurethane is present in the mixture in an amount ranging from about 12.5 to about 25 mole percent, the second polyurethane is present in the mixture in an amount ranging from about 25 to about 62.5 mole percent, and the third polyurethane is present in the mixture in an amount ranging from about 25 to about 50 mole percent.
Generally, the polyurethanes in the mixture are characterized by their end groups. One possible end group is the reaction product of a terminal isocyanate and a polyether alcohol, hereinafter referred to as the "polyether end group." Another possible end group is the reaction product of a terminal isocyanate and a reactant, so that this end group cannot further polymerize or participate in any further reactions once this reaction has occurred, hereinafter referred to as the "non-functional end group." The end groups on the polyurethane may be in any sequence and do not exclude the possibility that the polyurethane contains additional end groups such as being branched or star-shaped. For any end group that is the reaction product of a polyether alcohol and a terminal isocyanate, the polyether alcohol must have only one terminal hydroxyl moiety which can react with the terminal isocyanate so that the polyether end group cannot further polymerize or react after this reaction has occurred.
The polyether alcohol includes alkyl and aryl polyether alcohols. These alcohols may be straight or branched (C.sub.1 -C.sub.22) alkanol/ethylene oxide and alkyl phenol/ethylene oxide adducts, such as for example, methanol, ethanol, propanol, lauryl alcohol, t-octylphenol or nonylphenol ethylene oxide adducts containing 1-250 ethylene oxide groups. In addition, the polyether alcohol may also include alkanol/propylene oxide and alkyl phenol/propylene oxide adducts containing 1-250 propylene oxide groups. More preferred polyether alcohols in this invention include polyethylene glycol methyl ether and polypropylene glycol methyl ether. Most preferred polyether alcohols are polyethylene glycol methyl ethers with 15-50 ethylene oxide groups.
The non-functional end group is derived from a reactant such as an alcohol, amine, acid, mercaptan, and the like. It is preferred that the reactant is monofunctional in that it only has one group containing a hydrogen atom that can react with the terminal isocyanate group such as, for example, a monofunctional alcohol, monofunctional amine, monofunctional acid, or monofunctional mercaptan.
The monofunctional alcohol may include the alkyl alcohols (C.sub.1 -C.sub.40) such as methanol, ethanol, octanol, dodecanol, octadecanol, tetradecanol, hexadecanol, and cyclohexanol, and the phenolics such as, for example, phenol, cresol, octylphenol, nonyl and dodecylphenol. More preferred alcohols include the C.sub.14 -C.sub.20 alkyl alcohols, and a most preferred alcohol is 1-octadecanol.
The monofunctional amine may include both primary and secondary aliphatic, cycloaliphatic or aromatic amines such as the straight or branched chain alkyl amines, or mixtures thereof, containing about 1-20 carbon atoms in the alkyl group. Suitable amines include for example, n- and t-octyl amine, n-dodecyl amines, C.sub.12 -C.sub.14 or C.sub.18 -C.sub.20 n-alkyl and t-alkyl amine mixtures, and secondary amines such as N,N-dibenzyl amine, N,N-dicyclohexyl amine and N,N-dibenzyl amine.
The monofunctional acid may include, for example: C.sub.8 -C.sub.22 alkyl carboxylic acids, such as, for example, octanoic acid, decanoic acid, tetradecanoic acid, hexadecanoic acid, octadecanoic acid (stearic acid), eicosonoic acid, docosonoic acid; naturally occurring mixtures of acids, such as cocoa acids, tallow acids, rapeseed acids and the hydrogenated forms of these acids; aromatic acids, such as benzoic acid and napthenoic acids; alkyl substituted aromatic acids, such as octylbenzoic acid and dodecylbenzoic acid; alicyclic acids, such as cyclopentane carboxylic acid, cyclohexanecarboxylic acid, and cyclooctanecarboxylic acid; and alkoxypropyl acids derived from the Michael addition of alcohols of acrylic acid, such as 3-octyloxypropanoic acid, 3-dodecyloxypropanoic acid and 3-octadecyloxypropanoic acid.
The monofunctional mercaptan may include C.sub.1 -C.sub.30 mercaptans such as, for example, octyl mercaptan, decylmercaptan, dodecylmercaptan, tetradecylmercaptan, hexadecylmercaptan, octadecylmercaptan and the like.
The processes for the preparation of the polyurethane mixtures in this invention are well known and are illustrated in U.S. Pat. No. 4,079,028. The polyurethanes in the mixture can be prepared individually and then blended. It is preferred to prepare the polyurethane mixture in a one step process whereby all three polyurethanes are prepared simultaneously in the same reactor. The polyurethane mixtures are the reaction products of an organic diisocyanate; polyol, such as, for example, polyethylene glycol, polyether alcohol; and at least one reactant such as an alcohol, amine, acid, or mercaptan. The molar ratio of polyol to diisocyanate ranges from 1:1.01 to 1:5, preferably from 1:1.01 to 1:3. The moles of polyether alcohol and reactant must be at least two times greater than the difference between the moles of diisocyanate and polyol. The molar ratio of polyether alcohol to the reactant is from 10:1 to about 1:10, and more preferably from 1:1 to 1:5. The percent of each type of polyurethane in the mixture may be varied by changing the molar ratio of the polyether alcohol and reactant. A convenient reaction temperature is about 40.degree. C. to about 150.degree. C., preferably from about 60.degree. C. to about 130.degree. C.
It is preferable that the weight average molecular weight, Mw, of the polyether alcohol is greater than 500. It is also preferable that the weight average molecular weight, Mw, of the reactant, such as, for example, the monofunctional alcohol, monofunctional amine, monofunctional mercaptan, monofunctional acid, and the like, is less than 500.
The polyurethane mixture may be incorporated into aqueous compositions in amounts ranging from 0.005% to 20%, preferably from 0.01% to 10% and most preferably from 0.05% to 3.0% by weight of the aqueous composition. The polyurethane mixture may be mixed into the aqueous composition using conventional mixing equipment such as, for example, high speed dispersers, ball mills, sand mills, pebble mills, paddle mixers, and other such mixing equipment. The polyurethane mixture may be in the form of a dry powder, a premixed aqueous solution or a slurry or a solution in a water compatible solvent. In this regard, a solvent may be selected to prepare the polyurethane mixture so that it may be directly mixed into the aqueous composition. Of course, the composition may normally contain other known ingredients, such as, for example, pigments, surfactants, defoamers, preservatives, and the like, in known combinations and amounts depending on the particular end use.
Typical aqueous compositions which may include the polyurethane mixture of the present invention are paints, coatings, synthetic plaster, cosmetics, personal care items such as, for example, shampoos, hair conditioners, hand lotions, hand creams, astringents, depilatories, and antiperspirants, adhesives, sealants, inks, drilling fluids, packer fluids, topical pharmaceutical, cleaners, fabric softeners, pesticidal and agricultural compositions, and any other aqueous compositions requiring thickening. Usually these latex coating compositions contain added pigments, fillers and extenders such as, for example, titanium dioxide, barium sulfate, calcium carbonate, clays, mica, talc, silica, and the like.
Aqueous compositions thickened with the polyurethane mixture of this invention resist the tendency to sag. The sag resistance of a paint is measured as follows.